Now, to the crisis in Ukraine, and the American journalist held hostage by the pro-russian insurgents there. Hundreds of American troops are sent to eastern Europe in a show of support for our allies. ABC's Martha Raddatz is there in ington for us with the latest right now. Good morning, Martha. Reporter: Good morning, robin. American troops are arriving in Poland today to begin training exercises, with hundreds more in place in the baltic region by Monday. It is the most aggressive show of force from the U.S. Since the crisis began. 600 American soldiers immediately deployed to Poland, lithuan Lithuania, Latvia and Estonia, for infantry training exercises. And to reassure our allies and partners. The Pentagon making no secret that this is in response to Putin's aggression in Ukraine. If there's a message to Moscow, it's the same exact message. That we take our obligations very seriously on the continent of Europe. Reporter: A warship will be in the black sea. The "Uss Taylor" replacing "Uss cook." It comes hours after vice president Biden, who was in Ukraine, issued a stern warning to Putin. We have been clear that more provocative behavior by Russia will lead to more cost and to greater isolation. Reporter: Biden said, time is running out for Russia to move its tens of thousands of forces from Ukraine's eastern border. And its supporters out of government buildings in Ukraine. The U.S. Also wants to know the fate of this American journalist. That's him reporting from eastern Ukraine for vice, a rapidly growing news website, before he was arrested by unidentified people in uniform. This morning, pro-russian gunman say they are holding him. They say he is fine. But accuse him of suspected bad activities. They gave no indication when he would be released. Thank you so much. We see the tensions continue to escalate in that region.

This transcript has been automatically generated and may not be 100% accurate.

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